The present invention generally relates to monitor and safety circuits, and more particularly to an automatic recycle circuit for use in a process or apparatus of the type wherein process or equipment conditions are monitored to detect an abnormal condition the detection of which causes a temporary shut-down followed by an automatic re-start of the process or apparatus, the automatic recycle circuit being capable of permanently shutting down the process or apparatus if the detected abnormal condition has a duration exceeding a first predetermined time period or abnormal conditions are detected a predetermined number of times within a second predetermined time period.
In many processes and apparatuses it is normal to find electronic or other equipment that is subjected to transitory, non-damaging excursions of operating conditions. If the equipment is protected by monitor circuits which detect operation beyond set limits and automatically actuate shutoff circuits, it is quite often possible that the equipment could be immediately returned to service without damage to the equipment and a minimum downtime. With monitor circuits that are well thought out, sensitive, and fast acting, many damaging failures and externally induced problems can be reduced to non-damaging transients. In the cases of communications equipment or devices related to the safety or control of a process, equipment turned off and kept off by automatic monitor circuits is, at the very least, an inconvenience.
Recycle action has historically been performed in several ways. An operator can be employed to notice that the unit has shut-down and then manually restore operation. This operation is slow, not reliable if the operator's attention is distracted, expensive in that an operator must be paid a continuous salary, and cannot be used at all with unattended equipment. Circuitry using mechanical timers and relays has been used to provide recycle action, but such circuitry and mechanical timers are large, heavy, costly and use considerable amounts of power. Moreover, a system of this type is generally incapable of modifying its actions to adjust to different circumstances of failure. Specifically, in the case where the equipment has been damaged resulting in either the detected abnormal condition being permanent or occurring repeatedly, the recycle circuitry causes the process or apparatus to oscillate between shut-down and start-up conditions. In those processes and apparatuses which are controlled by computers, there exist the capability to recycle and to modify the recycle actions to adjust for different circumstances of process or equipment failure; however, computers are generally limited to very large, complex and expensive systems.